Nickel-hydrogen secondary batteries have been known as one type of alkaline secondary batteries. The nickel-hydrogen secondary battery typically employs as its negative electrode a hydrogen-absorbing alloy electrode utilizing a hydrogen-absorbing alloy.
Examples of the hydrogen-absorbing alloy for the negative electrode include Mm-based hydrogen-absorbing alloys comprising a Misch metal which is a mixture of rare earth elements, and Laves-phase hydrogen-absorbing alloys.
However, these hydrogen-absorbing alloys are generally susceptible to natural oxidation so that an oxide film and the like are formed on the surfaces thereof. Where an alkaline secondary battery employs as its negative electrode a hydrogen-absorbing alloy electrode formed of such a hydrogen-absorbing alloy, hydrogen gas cannot sufficiently be absorbed by the electrode because the hydrogen-absorbing alloy has a low activity in an initial cycle. This presents problems such as a reduced battery capacity and an increased battery inner pressure in the initial cycle.
To this end, a variety of methods have been proposed to recover the electrochemical activity of the hydrogen-absorbing alloy as a negative electrode active substance. One of the methods is an acid treatment wherein a hydrogen-absorbing alloy is immersed in an acid treatment solution such as containing hydrochloric acid to remove an oxide film from the surface of the hydrogen-absorbing alloy (Japanese Unexamined Patent Publications No. 4-179055 (1992), No. 5-225975 (1993), No. 7-73878 (1995), No. 7-153460 (1995) and the like). The acid treatment ensures a relatively excellent oxide film removing effect, and operations for the treatment is easy. Therefore, the acid treatment is an effective method for the recovery of the electrochemical activity of the hydrogen-absorbing alloy.
Where a hydrogen-absorbing alloy is subjected to the aforesaid conventional acid treatment, active sites appear on the surface of the hydrogen-absorbing alloy. However, the active sites of the hydrogen-absorbing alloy are extremely susceptible to oxidization, so that re-oxidization occurs during a battery fabrication process. Therefore, the conventional acid treatment fails to satisfactorily enhance the activity of the hydrogen-absorbing alloy electrode in the initial cycle. Accordingly, an alkaline secondary battery employing such a hydrogen-absorbing alloy electrode suffers an increase in the inner pressure during initial charging, a low initial discharge capacity, a poor low-temperature discharge characteristic, and a poor high-rate discharge characteristic.